Ultrafast quantum interferometry with energy-time entangled photons

TL;DR

This paper demonstrates ultrafast quantum interferometry using energy-time entangled photons with subpicosecond coherence times, achieving high-visibility interference and Bell inequality violation, opening new avenues for quantum metrology and communication.

Contribution

It introduces a method combining nonlinear optical gating with photon counters to observe ultrafast energy-time entanglement with high visibility and Bell violation.

Findings

Achieved 85.3% interference visibility in ultrafast regime

Measured a CHSH-Bell parameter of 2.42, violating local realism

Observed spectral and temporal interference patterns with subpicosecond coherence

Abstract

Many quantum advantages in metrology and communication arise from interferometric phenomena. Such phenomena can occur on ultrafast time scales, particularly when energy-time entangled photons are employed. These have been relatively unexplored as their observation necessitates time resolution much shorter than conventional photon counters. Integrating nonlinear optical gating with conventional photon counters can overcome this limitation and enable subpicosecond time resolution. Here, using this technique and a Franson interferometer, we demonstrate high-visibility quantum interference with two entangled photons, where the one- and two-photon coherence times are both subpicosecond. We directly observe the spectral and temporal interference patterns, measure a visibility in the two-photon coincidence rate of $(85.3\pm0.4)\%$, and report a CHSH-Bell parameter of $2.42\pm0.02$, violating the local-hidden variable bound by 21 standard deviations. The demonstration of energy-time entanglement with ultrafast interferometry provides opportunities for examining and exploiting entanglement in previously inaccessible regimes.